This invention relates to the design of imaging proportional counters (IPC) (also called multi-wire proportional counters or two-dimensional proportional counters) for imaging radiation from point sources located near the imaging device. An IPC is a radiation detector that uses gas as the detection medium and determines the position coordinates where radiation interacts with the fill gas.
Ionizing radiation and X-radiation imaging devices are useful in many fields such as X-ray diffraction, X-ray crystallography, nuclear physics, diagnostic radiology, nuclear medicine, DNA sequencing, etc. A well known X-ray imaging technique employs the photographic process. In this process photons (either visible light or X-ray) or ionizing radiation interact with the photographic emulsion to generate an image. Disadvantages of the photographic technique are the relative insensitivity of film, especially in X-ray applications, and the time required to develop and read film. Imaging proportional counters have been developed to produce good sensitivity and spatial resolution. Images can be collected directly in computer memory and processed quickly and easily.
Generally, imaging proportional counters have a gas filled envelope including a radiation pervious window. X-radiation or ionizing radiation traverses the window into the interior of the envelope. Ionizing radiation directly ionizes the fill gas leaving a trail of free electrons. X-radiation moves through the fill gas until it interacts with a fill gas atom producing a photo-electron. The photo-electron then moves a relatively short distance, ionizing the fill gas and producing a small cloud of free electrons. These free electrons created by the interaction of radiation in the fill gas are called primary electrons. Typically a few hundred primary electrons will be created. An electrostatic field is maintained in the region between the entrance window and a detector electrode assembly. Under the influence of this field the primary electrons drift toward the anode of the detector electrode assembly. As the primary electrons enter the high electric fields near the anode, amplification of the signal occurs. Several techniques are known for determining the position of the amplified signal. Generally, mutually orthogonal wire planes are used to collect charge and determine the location of the centroid of the amplified charge distribution.
If the measured location of the radiation event is to have a meaningful relation to the actual location of the primary event (if from an X-ray) or primary track (if from an ionizing particle) the primary electrons must drift to substantially the same location on the anode plane independent of the angle of the incoming radiation. This is a significant concern when imaging x-radiation since X-ray photons entering the detector at the same location and angle may penetrate the gas significantly different distances before interacting with a fill gas atom. This error in the measured position which is a function of the angle of the incoming radiation is called a parallax error.
A radiation camera disclosed in U.S. Pat. No. 3,786,270 to Borkowski et al. eliminates parallax errors when imaging X-rays from a point source by providing spherically symmetric electrostatic fields so that no matter where along a photon's trajectory within the interaction region a photo-electron is ejected, the resulting primary electrons drift to the same location at the anode of the detector. While the Borkowski radiation camera provides an elegant theoretical solution to the parallax problem, it has several drawbacks in actual application. The source of radiation must be located at a fixed, predetermined distance from the radiation pervious window of the detector. Thus, only a limited class of experiments can be performed with this camera. Furthermore, the Borkowski device requires spherically shaped wire mesh grid for its operation. Such a grid is difficult to fabricate so that it will retain its dimensional stability during operation of the radiation camera.
It is therefore an object of the present invention to provide a radiation detector which is capable of accurately imaging radiation from point sources located at varying distances from the apparatus.
Another object is the elimination of the need for a curved mesh focusing electrode within the detector.
A still further object of the invention is radiation camera which is simpler and more versatile than prior art detectors.